A torque tube transmits static and dynamic torsional loads in power transmission applications. For example, torque tubes are commonly used in aircraft to provide power transmission for actuating and braking flight control surfaces, e.g. flaps and slats, and for actuating aircraft door latches. Each end of a torque tube may be provided with an end fitting configured for mechanically coupling the torque tube to another transmission mechanism in the drive line.
So-called “composite” torque tubes made by winding carbon fiber tow around a mandrel are widely used in the aircraft industry in part because they are lighter than metal torque tubes yet still meet critical strength requirements. Composite torque tubes also offer improved flexural rigidity and torsional rigidity.
A recognized challenge in manufacturing composite torque tubes involves the attachment of the metallic end fittings to the composite material tube portion. It is important to attach the metallic end fittings to the composite tube such that the junction between the composite material and each end fitting is reliable and not prone to failure under loading.
Various schemes for attaching end fittings to a composite torque tube have been proposed. U.S. Pat. No. 7,335,108 (Lin e al.) describes a drive shaft assembly having a composite tube and a pair of end fitting captured at each end of the composite tube. The assembly may be made by winding pre-impregnated (“prepreg”) fiber tow over a mandrel and the end fittings, enclosing the components in a vacuum bag and evacuating the internal space of vacuum bag, curing the prepreg fiber tow, and removing the vacuum bag and mandrel. Each end fitting includes a neck leading to a region having radially projecting lugs angularly spaced about a central axis of the assembly. The region having radially projecting lugs is captured by the composite material to transmit axial and torsional loads between the composite tube and the end fitting. The composite material has a non-circular cross-sectional profile where it surrounds the lug region of the end fitting.
U.S. Pat. No. 7,419,435 (Borges et al.) discloses a composite torque tube assembly formed by mounting a metallic end fitting on a mandrel and wrapping a portion of the end fitting and the mandrel in composite material which is then cured. The end fitting is characterized by one or more pairs of diametrically opposite lobes extending radially outward from a central axis of the end fitting to improve torque transmission between the composite material and the end fitting. The end fitting includes a cylindrical axial passage aligned with a cylindrical axial passage of the tube formed of composite material, wherein the diameter of the tube passage is greater than the diameter of the end fitting passage. Thus, the forming mandrel must have a radial step transitioning from the main diameter of the composite tube to the smaller diameter of the end fitting. Consequently, the mandrel cannot be axially withdrawn from the assembly after curing, and instead must be dissolved by flushing a dissolving liquid through the passage of the end fitting. This process is expensive and harmful to the end fitting and the composite material.
U.S. Pat. No. 7,682,256 (Brace et al.) teaches a composite torque tube assembly wherein a metallic end fitting is provided with an external coupling region having a plurality of radially-protruding lugs spaced apart from one another to define channels in between rows of lugs. The channels are arranged to extend at an intended fiber wrap angle to directly receive the wound composite material to enhance torque transmission between the composite material and the end fitting. The lugs have abrupt transitions and/or sharp edges that may degrade the composite material where it interfaces with the end fitting. Moreover, torque transmission is distributed very unevenly among the lugs, with a leading lug in each row receiving most of the transmitted force. As a result, the torque tube assembly is prone to having a reduced service life.
U.S. Pat. No. 7,874,925 (Dewhirst) describes a method of making a torque tube assembly by pressing an internally-splined end fitting onto an end region of a cured composite tube. The end region of the composite tube must be precision-machined to a predetermined diameter to provide a slight interference fit with the internally splined end fitting. An adhesive is introduced when the end fitting is pressed on to the composite tube to improve bonding. The process is expensive and requires significant operator skill to yield an acceptable end product.
U.S. Pat. No. 8,025,580 (Genot et al.) discloses a composite torque tube assembly having a metal fitting arranged fixedly around a portion of a composite tube. The metal fitting has a non-circular inner surface profile mating with a complementary outer surface profile of the composite tube in the region of the metal fitting. The portion of the composite tube surrounded by the metal fitting is manufactured directly inside the metal fitting by providing a fibrous preform inside the fitting and injecting a plastic, such as epoxy resin, into the fibrous preform to form a matrix. The manufacturing method does not involve winding uncured prepreg fiber tow over a mandrel.
What is needed is a composite torque tube assembly that is more economical to manufacture than known composite torque tube assemblies, yet meets specified performance requirements.